1. Field of the Invention
The present invention relates to a drive control device for a servo motor, in a machine tool, industrial machine, or the like, which is driven and controlled by means of a control device, such as a numerical control device. In particular, it relates to learning control used when the same operating pattern is implemented repeatedly, in such a manner that the same pattern is processed repeatedly by means of positional commands of a prescribed uniform pattern.
2. Description of the Related Art
Learning control is already known as a method for increasing processing accuracy and making control deviation converge to zero, in cases where processing, or the like, is carried out by repeatedly instructing the same pattern of commands. This learning control involves setting the time period of a pattern operation, such as one rotation of a workpiece, or the like, as a learning cycle, rotating the workpiece for a plurality of times and determining the positional deviation at each prescribed control cycle, and storing correction data in a memory on the basis of this positional deviation, in such a manner that the positional deviation can then be made to converge to zero by adding, to the positional deviation at each control cycle of the current pattern cycle, the correction data at corresponding control cycle of the previous pattern cycle stored in the memory (see, for example, Japanese Patent Application Laid-open Nos. H4-362702 and H6-309021.
Moreover, a control method for linearly moving axes is also known which involves repeated control whereby, when processing a workpiece installed on a rotating axis by means of positional commands repeated at periodic intervals in synchronization with the rotating axis, using a tool installed in a linearly moving axis, the position of the linearly moving axis in correspondence with the position of the rotating axis at each prescribed sampling cycle is determined by means of a table and taken as a positional command for the linearly moving axis, in addition to which, correction data for one repetition cycle is determined and stored, on the basis of the positional deviation of the linearly moving axis, and, to the sampled positional deviation, the corresponding correction data for the previous repetition cycle is added to obtain a positional command (see Japanese Patent Application Laid-open No. H4-323705).
In the inventions disclosed in Japanese Patent Application Laid-open Nos. H4-362702 and H6-309021, mentioned above, correction data for one cycle of the learning control is stored in correspondence with the sampling time within each cycle, and therefore, if the command velocity changes, the learning cycle also changes, and hence the correction data obtained already by means of learning control will become unusable, and it will become necessary to create correction data again. Moreover, if the command velocity changes and there is no periodicity in that change, then the correction data obtained by learning control will be obtained in the form of a time function, and consequently the correction data obtained for a previous pattern cycle will not correspond to the correction data for each respective control cycle in the current pattern cycle, and will therefore be unusable.
The commands instructed in each cycle of the same pattern are positional commands, and these commands form a pattern corresponding to positions. However, in the memory storing correction data for one pattern cycle in learning control, the correction data for each prescribed control cycle, such as a position or velocity control cycle, is stored for one pattern cycle, and the oldest data of the correction data for each respective control cycle stored in the immediately previous pattern cycle, in other words, the correctional quantity based on the correction data of the previous pattern cycle, is added to the positional deviation, but since the speed of the motor varies, the correction data for the previous pattern cycle to be added to the current positional deviation does not correspond to the position of the drive object, such as the workpiece, or the like. As a result of this, the positional deviation does not converge to zero.
Moreover, as in the invention disclosed in Japanese Patent Application Laid-open No. H4-323705 mentioned above, in a method where, in the case of a rotating axis and a linearly moving axis driven in synchronization, correction data for the linearly moving axis is stored for one cycle of a repeated cycle, in correspondence with the position of a rotating axis, and the positional deviation of the linearly moving axis is reduced by correcting the sampled positional deviation by means of the correction data, then since the correction data is stored in accordance with a position of the rotating axis, it will still be possible to use the correction data effectively for repeated control and thereby to reduce the positional deviation, even if the velocity changes. However, in the invention disclosed in Japanese Patent Application Laid-open No. H4-323705, a conversion table is required for obtaining the position of the linearly moving axis from the feedback position of the rotating axis, and if the pattern for one pattern cycle has been changed, or the like, then a new conversion table of this kind must be created, which makes this method difficult to apply when processing various types of patterns, or the like.